To receive terrestrial television transmissions, especially digital video broadcasting (such as DVB-T and DVB-H), in a portable or mobile environment, a compact omni-directional wideband antenna covering the UHF TV band (470-860 MHz) is desirable. Reference herein to “wideband” is meant to refer to a situation in which a desired tuning range of the antenna is many times the bandwidth of the antenna. Such an antenna is difficult to implement, especially if an antenna having minimal gain is desired.
It is conventionally considered easier to construct a narrowband antenna having, for example, a dipole, loop, patch, or meander structure. Such antennas can be tuned either electrically (using variable reactance) or mechanically (by varying the length of the elements) to cover the desired band. U.S. Pat. No. 6,606,069, for example, describes an electrically tuned antenna.
However, narrowband antennas must accurately track a desired signal frequency in order to avoid loss of gain. This can be difficult because a variety of factors can easily or rapidly cause detuning (such as the presence of a person, metallic objects, and the like in proximity to the antenna).
Conventional transceiver systems (such as short-wave marine radios) having a tunable antenna often use transmit mode tuning. That is, the antenna is tuned according to a signal being transmitted. More specifically, the antenna is tuned to achieve a maximum return loss (or minimum voltage standing wave ratio (“VSWR”)). In the receive mode, the antenna keeps the same tuning as in the transmit mode. JP 60 097 728 generally describes a non-automatic system of this type.
If, however, the antenna becomes detuned in receive mode with respect to a received signal, the tuning cannot be readjusted to compensate without going into transmit mode. It will be appreciated that this issue becomes significant in many applications (such as transporting digital information (data, video, etc)) because the received signal is continuous and there is no feasible period when the receiver can be disconnected to allow retuning in the transmit mode. In particular, television receiver radiation emissions are strictly regulated such that applying even a very small signal into the antenna for transmit mode tuning is unlikely to be permissible.